Re: Inexpensive HV probes

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>

By using two of these dividers in series, with your meter across the pair
of 100K resistors, adding a 200K resistor across the two hundred K
resistors, and grounding the middle point, you have a MUCH safer apparatus.
 The existing circuit shown will float the meter at basically the voltage
of the right hand terminal.  If you are measuring to ground (i.e. grounding
the right hand terminal), the meter is at ground potential, BUT, if the
ground wire comes loose, the whole thing floats up to HV. Bad things can
also happen if other wires come loose.  With a pair of dividers, even if
one of the grounds comes off, or you lose a lead, the meter case is held
close to ground.  

A typical DVM is probably insulated for 1000V relative to ground, so you
should look at your circuit and consider ALL possible failures (shorts or
opens), and see if there is any way your meter can wind up at more than 500
or so volts relative to ground.


HV --- big resistor -- 100K - ground - 100K ---- big resistor -- other HV
                     |                       |
                     -------- 200 K ----------
                     |                       |
                     ---- meter --------------


Another strategy is to get some of the low voltage sealed spark gaps like
they use in surge suppressors (or, even, neon bulbs).  Hook them up from
each meter lead to ground, and if something does go wrong, they'll clamp it
to a safe voltage.

Be particularly aware of what will happen if a fuse blows.  It's easy to
have a fuse blow and then have your entire system wind up floating at HV.

Tesla list wrote:
> 
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<Mddeming-at-aol-dot-com>
> 
> In a message dated 4/26/01 6:49:53 PM Eastern Daylight Time, tesla-at-pupman-dot-com
> writes:
> 
> >
> > Original poster: "jpeakall by way of Terry Fritz <twftesla-at-uswest-dot-net>" <
> > jpeakall-at-mcn-dot-org>
> >
> > I need to get a HV probe for my DMM so I can test transformers. Anybody
know
> > of some good sources or a homemade version?
> >
> > TIA
> >
> > Jonathan Peakall
> 
> Hi Jonathan!
> I just finished making one. I made a string of 10 10Meg 1/2W 5% resistors
> with a 100K 1/2 W on one end. input leads across the whole string,
> scope/meter across the 100K only. Very close to a 1000:1 voltage divider.
> Slid the whole thing into a piece of poly tubing with scope leads coming out
> a split on the 100K end.
> The probe was then put into a 1/2" PVC pipe with end caps and 2 side holes
> for meter leads for rigidity:
> 
> HV    ------------------------------------------------- HV
> <======10M-10M-10M-10M-10M-10M-10M-10M-10M-10M+100K=+=====>
>      -----------------------------------------|-----|-
>                                               |     |
>                                               V     V
>                                            to meter/scope
> Hope this helps.
> Matt D.